Recently, a fuel cell unit provided with a power generation unit which generates power upon chemical reaction and auxiliaries which assists an operation of the power generation unit has been known (for example, PTL 1). In a normal operation, output power (i.e., generated output) of the fuel cell unit is controlled to follow power consumption of a load provided on a power line which connects a grid and the fuel cell unit (i.e., load following control).
By the way, as an operation mode of the fuel cell unit, known is an operation mode for controlling output power of the power generation unit so that output power of the power generation unit becomes significantly the same as power consumption of the auxiliaries (hereafter, referred to as an idling mode) (for example, Patent Literature 2 and Patent Literature 1). For example, the idling mode is a mode for continuing the operation of the fuel cell unit in such a case where a power demand in a load is temporarily low.